A navigation system guides a user to a desired destination with a map shown on a navigating display. In response to a user's request, map data stored in a server placed at a map database site are transmitted to such a navigating display as an on-vehicle navigation unit, mobile navigation unit, PDA (personal digital assistants), or mobile device, through communication means. The communication means is composed of a computer network (the Internet, for example) or a mobile communication network. The navigating terminal includes a GPS (global positioning system) to detect a vehicle's current position, and provides a map on a display thereof using both the vehicle's current position and the received map data.
Such a navigation system is advantageous in that it enables each navigating display to make reference to the newest versions of maps. However, this requires reloading the newest map data stored in a block in the navigating display whenever the map data handled in the server is updated or the user moves beyond the currently loaded map data.
A navigating display participating in the navigation system downloads all the map data in one block to renew the map data when the map data is updated in the server. In some conventional navigation systems, even when a unit of map data is partially updated in the server, the entire map data in each block is downloaded. As a result, amounts of data being received and reloaded become extremely large and impose a high communication load on a communication line. The large amount of data necessitates a longer time to process map data in the reloading operation.